The present invention relates to a scanning exposure method and a circuit element producing method which employs the scanning exposure method. More particularly, the present invention relates to a scanning exposure method which is used in a lithography process for producing semiconductor elements such as ICs, and also relates to a circuit element producing method which employs the scanning exposure method.
Exposure systems play an important role in lithography processes for producing semiconductor elements (e.g. ICs). The mainstream of such exposure systems is known as a stepper, which forms a plurality of shot regions (i.e. pattern regions which are each formed by one exposure shot) on a wafer coated with a photosensitive material such as a resist by repeating a process in which a mask pattern is projected onto the wafer, and the wafer is stepped (stepwisely moved). Each of a plurality of shot regions formed on the wafer constitutes an individual semiconductor element as a product. Referring to one of the shot regions formed on the wafer. A plurality of masks (1 to n masks) each having a predetermined pattern written thereon are prepared, and overlay exposure is carried out a plurality (n) of times using each of the masks, thereby producing one semiconductor element. Accordingly, the photosensitive substrate has a plurality (n) of wiring layers formed thereon. More specifically, a pattern formed on a first mask (i.e. a mask for a first layer) is projected onto a photosensitive substrate coated with a resist, and then a developing process is carried out to form a first-layer pattern on the photosensitive substrate. Next, a pattern formed on a second mask (i.e. a mask for a second layer) is projected over the first-layer pattern formed on the photosensitive substrate. Then, a developing process is carried out to form a second-layer pattern on the photosensitive substrate. Next, a pattern formed on a third mask (i.e. a mask for a third layer) is projected over the second-layer pattern formed on the photosensitive substrate, and then a developing process is carried out to form a third-layer pattern on the photosensitive substrate. By repeating such a projection and developing process n times, a semiconductor element comprising n layers is produced.
An alignment technique whereby a pattern of a mask which is used for the present exposure process is overlaid on a pattern (i.e. a pattern of a mask used in the preceding exposure process) formed on a photosensitive substrate is regarded as one of the most important techniques in lithography processes.
Alignment methods may be roughly divided into two categories: die-by-die alignment (hereinafter referred to as "D/D alignment", and global alignment.
In the D/D alignment, for each exposure shot, the position of an alignment mark provided in a previously shot region is measured with an alignment sensor, and a projected image of a mask pattern and the shot region are aligned with each other on the basis of the alignment mark.
In the global alignment, positions of alignment marks formed in a predetermined number of shot regions for alignment (i.e. sample shot regions) among a plurality of shot regions formed on a photosensitive substrate are measured with an alignment sensor, and a substrate stage having the photosensitive substrate placed thereon is moved on the basis of the result of the measurement. In the global alignment, the substrate stage needs to be accurately positioned; in the existing exposure systems, the position of the substrate stage is measured with a laser interferometer, and the movement of the substrate stage is controlled on the basis of the result of the laser interferometric measurement. The positional relationship between the alignment sensor and the mask pattern has previously been determined. Therefore, the mask pattern and the pattern (shot pattern) formed on the photosensitive substrate are indirectly aligned with each other by means of global alignment. Among the global alignment methods is an enhanced global alignment (EGA) method. In the EGA method, an array error of shot regions is obtained on the basis of positional information on several to ten sample shot regions. Then, new shot array coordinates are calculated by using a statistic computing technique, and the substrate stage is positioned according to the new shot array coordinates. The details of the EGA method are disclosed in U.S. Pat. No. 4,780,617.
Recently, there has been a tendency for each individual chip pattern of semiconductor elements to increase in size, and therefore, there has been a demand for an exposure system designed to transfer a larger mask pattern by a single exposure operation. To meet such a demand, scanning exposure systems have been proposed. In a typical scanning exposure system, a mask, which is being illuminated, is scanned in a first direction, and a photosensitive substrate is scanned in a second direction corresponding to the first direction in synchronization with the scanning of the mask, thereby transferring the mask pattern onto a shot region on the photosensitive substrate. Attention is being given, particularly, to what is called a step-and-scan type scanning exposure system, in which a mask pattern is sequentially transferred onto shot regions on a photosensitive substrate by repeating scanning exposure and stepping operation (refer to, e.g., U.S. Pat. Nos. 5,194,893 and 4,924,257).
Incidentally, as semiconductor elements are increasingly reduced in size and line width, the alignment accuracy required for the above-described exposure systems has become increasingly severe, and as the required alignment accuracy becomes increasingly strict, the tolerance of alignment becomes an extremely small value.
In the above-described step-and-scan type exposure system in particular, when a mask stage for mounting a mask and a substrate stage for mounting a substrate are moved for scanning, the center of gravity of equipment (including the mask stage, the substrate stage, the carriage for supporting the mask stage and the substrate stage, the vibration isolating table, the measuring system, etc.) shifts, and as the alignment accuracy becomes increasingly severe, the amount of shifting in the center of gravity has become of influence. The amount of distortion of the system caused by a shift in the center of gravity, the frequency of the system, the difference in resistance according to the scanning direction, vibration, etc. have also become of influence.
In the step-and-scan exposure system, if the scanning direction is assumed to be along the direction of the Y-axis, there are two different types of shots on a wafer: shots which are made when the substrate stage is scanned in the direction +y; and shots which are made when the substrate stage is scanned in the direction -y.
FIG. 3 in the accompanying drawings shows a layout of first shot regions (i.e. first-layer shot regions) formed on a wafer W by the step-and-scan exposure method. In the figure, the solid-line arrows indicate the direction (scanning direction) of movement of the wafer W during scanning exposure, and the dotted-line arrows indicate the direction of stepping of the substrate stage (wafer W).
Incidentally, mechanical conditions of the exposure system are different for the movement of the substrate stage in the direction +y than for the movement of the substrate stage in the direction -y. Therefore, it is not necessarily possible to say that the movement of the substrate stage in the direction +y and the movement of the substrate stage in the direction -y are completely compatible with each other in terms of exposure position. In other words, the scanning in the direction +y and the scanning in the direction -y each reflect a mechanical tendency peculiar to the system.
Accordingly, if the scanning exposure direction of the substrate stage in the preceding (first) exposure process and the scanning exposure direction of the substrate stage in the present (second) exposure process differ from each other, the alignment accuracy is reduced. This problem was found by the inventor of the present application.